Measurements

Signal acquisition. All data, force and EMG were collected with AD-converter (Power 1401, Cambridge Electronic Design, UK) at 1000 samples per second. Analyses were performed with Spike 2 v 5.21 software (Cambridge Electronic Design, UK).

Maximal voluntary contraction and rate of force development. Maximal voluntary contraction measurements (MVC) for right plantar flexors were performed in a custom made dynamometer (University of Jyväskylä, Finland). During the tests, subjects were fixated to a bench with 4-point seat belt. Their right knee was fully extended and in a 180°

angle and ankle was in a 90° angle to tibia. Left leg was kept relaxed on the left side of the bench, so that it did not touch the dynamometer. Subjects were instructed to push against the force plate as hard and fast as possible for 3 seconds whilst they were verbally

encouraged by the instructor. Minimum of three attempts were made or until improvement of the MVC was less than 5% from the second best attempt with 45-s-rest period between the trials. Maximal force was analysed from the best trial as a peak-to-peak force value from the onset of the force production to the highest force level. RFD was analysed for the best trial ±50 ms around the steepest point from the early phase (0-200ms) of the force production.

Vertical jumps. Countermovement jump (CMJ) and static jump (SJ) without arm swing were performed on force plate (AMTI, Massachusetts, USA). Subjects were instructed to keep their arms on their hips while performing the vertical jumps. Knee angle was instructed to be 90°. On SJ subjects were instructed to stay on correct knee angle for a second before jumping. On CMJ subjects started from a standing position and lowered to 90° as fast as possible and jumped from that position in a one fluid motion. Each jump was performed three times with 45-second rest interval. Best flying time was analysed from force plate data and jump height was calculated (Moir 2008). Test instructor supervised all the jumps and if 90-degree angle was not achieved jump was marked as failed.

Skating speed and acceleration. Skating sprint and acceleration were used to measure sport specific training adaptation. Measurements were made in standard sized ice hockey rink (Figure 1) with full kit excluding the stick (to prevent false triggering of the photocells). Photocells (Newtest, Ele-Products Oy, Tyrnävä, Finland) were placed on goal line (start), defending faceoff spots outer line (acceleration) and on attacking zones blue line (speed). Three attempts were made for each subject and the best acceleration (11 m) and start to finish (34 m) times were recorded with Powertimer (Newtest, Ele-Products Oy, Tyrnävä, Finland) measurement system.

Figure 1 Photocell placement on skating tests. Acceleration was measured from the start to the defending faceoff sports outer line and speed was measured from start to finish.

Electromyography. Surface electromyography (EMG) from gastrocnemius lateralis, soleus, tibialis anterior, biceps femoris and vastus lateralis were recorded with bipolar electrode (AMBU BlueSensor N, Copenhagen, Denmark) with 2 cm inter-electrode distance. Electrodes were placed according to SENIAM recommendations (Hermens et al.

1999). Before the placement, the skin was abraded with sand paper and cleaned with alcohol. If electrode impedance was higher than 8kΩ the preparation was repeated. EMG (RMS) signals were band pass filtered (10 to 500 Hz), amplified (gain 1000) and sampled at 1500 Hz with Telemyo 2400R (Noraxon, Scottsdale, AZ, USA) system. RMS values of 0-500ms (RFD) and 500-1000ms (MVC) from muscle contraction onset were analysed from soleus, tibialis anterior and gastrocnemius lateralis. Tibialis anterior activity during plantar flexion was then analysed relative to tibialis anterior dorsiflexion MVC EMG-RMS value to analyze changes in muscle coactivation (Simoneau et al. 2006). Due to technical problems group sizes for analyzed EMG-RMS results (RFD, MVC and coactivation) dropped to 5 subjects per group.

Electrical stimulation. H-reflex and M-wave during standing at rest were measured from right soleus muscle to determine motor neuron excitability and presynaptic inhibition changes caused by HIIT. An oval 5.1×10 cm anode (V Trode, Mettler Electronics Corp., Anaheim, USA) was placed superior to patella. Constant current stimulator (DS7A, Digitimer, UK) was used to stimulate the right tibial nerve in the popliteal fossa with a cathode using 200 µs square pulse. Correct positioning of the cathode was confirmed by ramping up the stimulation amplitude until H-reflex and M-wave were visible. Optimal stimulation site was determined by moving a temporary cathode laterally to pinpoint position for the highest M-wave (peak-to-peak) amplitude. The temporary cathode was then replaced with a permanent 3x2 cm electrode (AMBU BlueSensor N, Copenhagen, Denmark) that was used during all the tests. A constant pressure was applied to the stimulating electrode throughout the session.

Next, stimulation intensity was lowered until no visible response was elicited (typically around 10 mA). Stimulation was then increase in steps of 1 mA until H-reflex was abolished

and the maximal H-reflex (Hmax) was measured with the intensity that elicited highest peak-to-peak value. Stimulation intensity was further gradually increased 5 mA steps until M-wave peak-to-peak value was saturated. Stimulation intensity was then increased with 50

% to supramaximal level (Msup) to ensure maximal M-wave (Mmax). Hmax was normalized with Mmax to be able to compare the results between test sessions (different days). At least 8 second inter-stimulus interval was used to avoid effects of post activation depression (Crone

& Nielsen 1989).

To estimate of excitability of α-motoneuron pool and level of presynaptic inhibition, H-reflex measurements were superimposed to MVC. Stimulation intensity was 20% ± 2.5% of of the supramaximal intensity used in Mmax-recording. This submaximal M-wave peak-to-peak amplitude was monitored online on a screen to constantly modify stimulation intensity to keep the M-wave amplitude sable. This ensured constant amount of motor neurons recruited between the pre- and post-training test session, and thus H-reflex/Mmax20% values can be compared between sessions. Ten attempts were made to get minimum of four successful measurements that were within 17.5%–22.5% of the supramaximal Mmax. Peak-to-Peak values of H-reflexes and M-waves were then analyzed from the accepted samples, and were averaged prior computing H-reflex/Mmax20% ratio.

V-waves were recorded to quantify changes in the level of efferent motor drive during the intervention. Subjects were instructed to make similar efforts as in MVC measurements.

After MVC force reached its plateau stimulation with supramaximal intensity used in Mmax -recording was given to peripheral nerve to elicit M- and V-wave responses. Eight attempts were made and values from attempts that reached ≥ 90% of measured MVC were analyzed (minimum of 2 per subject). Peak-to-peak values from both responses were averaged and V-wave value was then normalized to M-V-wave.